1. Field of the Invention
The present invention relates to a method for colored image formation by which a colored image is formed on a lithographic printing plate precursor. More specifically, the invention relates to a method for forming a colored image having good visibility on a lithographic printing plate precursor, and in particular, to a method for colored image formation to form a colored image having good visibility on a lithographic printing plate that is capable of being developed on a printing press.
2. Background Art
A lithographic printing plate in general consists of an oleophilic image area of receiving an ink in the printing process and a hydrophilic non-image area of receiving a fountain solution. Conventional lithographic printing plates are usually produced by mask-exposing a PS plate which has a layer of an oleophilic photosensitive resin provided on a hydrophilic support through a lith film, and then removing by dissolution the non-image area in a developer.
In recent years, image is electronically processed, saved and output by a computer as digital information. Thus, the processing of image formation in accordance with digitized image information is preferably carried out such that an image is directly formed by scan-exposing lithographic printing plate precursor using a highly oriented active radiant ray such as laser light, with no intervention of a lith film. This technique of plate-making a printing plate from digitized image information as such with no intervention of a lith film is referred to as the computer-to-plate (CTP) technique.
When the method for making a printing plate by means of a conventional PS plate is attempted in the way of the CTP technique, there is a problem that the wavelength range of the laser light and the photosensitive wavelength range of the photosensitive resin do not match.
Further, in a conventional PS plate, the step of dissolving and removing the non-image area after exposure (development processing) is indispensable. Moreover, there has been required a post-treatment step of washing the developed printing plate, treating the plate with a rinsing solution containing surfactants, or treating the plate with a desensitizing solution containing gum arabic or a starch derivative. This problem of necessitating such additional wet processing has been a significant problem for the conventional PS plate to be solved. Even though the first half of the plate-making process (image forming process) has been simplified by the digital processing, the effect of simplification is still insufficient with the later half involving the complicated wet processing (development processing).
In particular, consideration for global environment is recently a great concern to the entire industry. In view of consideration for global environment, too, the post-treatment involving wet processing should be preferably simplified or modified to dry processing.
Therefore, as one way of dispensing the treatment steps, a method called on-press development has been proposed, wherein an exposed printing plate precursor is loaded on the cylinder of a printing press, and a fountain solution and an ink are supplied while rotating the cylinder, to remove the non-image area of the printing plate precursor. That is, it is a method in which an exposed printing plate precursor is loaded on a printing press as such, and the processing is completed in the course of conventional printing operation.
The lithographic printing plate precursor suitable for such on-press development is required to have a photosensitive layer which is soluble in a fountain solution or an ink solvent, and to have lightroom handlability appropriate for developing on a printing press placed in lightroom.
It has been substantially impossible to satisfy such requirements with the conventional PS plates.
Therefore, in order to satisfy such requirements, there has been proposed a lithographic printing plate precursor having a photosensitive layer in which fine particles of a thermoplastic hydrophobic polymer are dispersed in a hydrophilic binder polymer, provided on a hydrophilic support (for example, see Patent Document 1: Japanese Patent Laid-Open No. 2001-277740). The printing precursor can be subjected to image formation through coalescence (fusion) of the thermoplastic hydrophobic polymer fine particles with the heat generated by photo-thermal transition upon exposure to an infrared laser during the plate-making process, subsequently loaded on the cylinder of a printing press, and then on-press developed supplying at least one of a fountain solution and an ink. Since this lithographic printing plate precursor has its photosensitive band in the region of infrared, it shows handlability in lightroom.
However, the image formed through coalescence (fusion) of the thermoplastic hydrophobic polymer fine particles has insufficient strength and thus has a problem in the press life as a printing plate.
Further, in place of thermoplastic fine particles, a lithographic printing plate precursor containing microcapsules which encapsulate a polymerizable compound has been proposed (for example, see Patent Documents 2 to 7: Japanese Patent Laid-Open No. 2000-211262, Japanese Patent Laid-Open No. 2001-277740, Japanese Patent Laid Open No. 2002-29162, Japanese Patent Laid-Open No. 2002-46361, Japanese Patent Laid-Open No. 2002-137562, and Japanese Patent Laid-Open No. 2002-326470, respectively). The printing precursor according to such proposal is advantageous in that the polymeric image formed by reaction of the polymerizable compound has higher strength than the image formed by fusion of fine particles.
In addition, since the polymerizable compound has high reactivity, there have been proposed a number of methods to segregate the compound using microcapsules (for example, see Patent Documents 2 to 7). It has been also proposed to use a thermodegradable polymer for the microcapsule shell.
However, with the lithographic printing plate precursor of the background art as described in Patent Documents 2 to 7, it has been difficult to confirm the image formed by exposure to laser light on the printing plate. For this reason, there has been the possibility of having a problem that the top and the bottom of the printing plate is reversed on the printing press, or that it is not known whether there would be a displacement in the image until printing is completed. Thus, it is desired to further improve the visibility.